1. Field of the Invention
This invention relates to direct view displays and more specifically to a paper white display that combines passive addressing technology with a bistable micromirror faceplate.
2. Description of the Related Art
Direct-view displays produce images that can be viewed directly without the aid of magnification or projection. The market for direct view displays spans a continuum of performance and price that includes the ultra high performance but very expensive flat-panel DTVs, moderately performing and priced laptop computers, and the lower performing but much cheaper personal digital assistants, electronic books and cellular telephones. The high-end displays offer high spatial and color resolution but are very expensive and consume a lot of power. The low-end displays offer less resolution but are relatively cheap and can be operated from battery power.
This low-end market is currently dominated by the multiplexed liquid crystal display (LCD) technology. Multiplexed LCDs sacrifice grey scale performance in favor of fabrication simplicity and power consumption by eliminating the thin film transistor (TFT) array used in Active Matrix LCDs (AMLCD), which dominate the laptop computer market. The liquid crystal panel is fabricated with orthogonal row and column addressing lines on opposite sides of the liquid crystals that are driven by row and column drive electronics. The row drivers enable the row addressing lines one row at a time while the column drivers apply selected voltages to all of the column addressing lines to apply a voltage across the cells in the enabled row. The voltage changes the transmissive characteristics of the liquid crystal, which in turn optically modulates the amount of light transmitted through the LCD.
Because liquid crystals respond relatively slowly to changes in the applied voltage, the cell modulation is proportional to the root-mean-square (rms) voltage applied across the cell throughout the frame time. Although the voltage applied during the row enable is very large, the background noise created by the applied voltages for the remaining n-1 rows greatly reduces the RMS value of the margin between the off-state and full on-state of the liquid crystal. For example, commercially available AMLCDs can resolve about 16 million different colors while similarly available multiplexed LCDs can resolve only 256 different colors. As the number of scanned rows increases, this disparity in grey scale color resolution grows.
These LCDs must be constantly refreshed, e.g. 30 times per second, which consumes a lot of power. Without a sustaining voltage they will decay from their modulated state to their relaxed state over time. Furthermore, the polarizers inherently required by LCDs absorb such a large fraction of the ambient light, typically 60%-70%, they are unable to produce the "paper white" quality desired by the industry. As such consumers must make do with cell phones and PDAs whose gray displays are difficult to read even under the best ambient lighting conditions. Power consuming backlights must be added to improve their readability to minimum acceptable levels.
Another class of displays that are prevalent and gaining market share in low-end applications are bistable displays. True bistable displays have two stable states, black and white, and require no power to remain in either state. Since bistable displays do not require continuous refreshing they are very low power. This makes them ideally suited for applications such as cellular telephones and PDAs. Adequate grey scale resolution can be achieved using standard half-toning techniques. However, known bistable displays suffer from the same problem as multiplexed LCDs, their white state tends to be gray rather than paper white. As a result, they require backlighting and their readability is limited even in the best ambient light conditions.
Kent Displays, Inc. is the leader in bistable Cholesteric LCDs. The bistability of cholesteric optical textures allows for high resolution on a low cost passive matrix with reduced power consumption since power is not needed to continuously refresh the image. The reflected colors of the cholesteric liquid crystal materials provide for a display that is readily viewed in sunlight or low ambient light without dedicated illumination. However, single layer cholesteric LCDs are colored and combining different color layers to get a neutral color dark state severely reduces the overall brightness of the display. These displays are very dim; black characters on a dark grey background instead of black on a white background.
Xerox PARC is developing a gyricon technology in which 100 micron bichromal spheres are cast in a clear elastomer on a flexible substrate. The sphere dipole causes rotation in an electric field to show either the black or white surface of the sphere. The gyricon display is thin, flexible, exhibits a wide viewing angle and, like other bistable devices, requires no power to store the device. However, contrast ratios of only 6:1 have been achieved.
E Ink, Corporation is developing an alternative bistable display technology, electronic ink, in which the ink is made of microcapsules, each of which can change color with an applied electric field. More specifically the microcapsules are filled with a colored dye. Charged white particles are suspended in the dye. Orienting the electric field the right way causes white particles to be attracted to the surface so that the display appears white and vice-versa. E Ink claims to have achieved 75% brightness, 30:1 contrast ratio and a 180 degree viewing angle.
Iridigm Display Corporation uses a MEMS technology in which bridge-like elements move up and down in response to an applied voltage to achieve a bistable display. By changing an element's position from up to down, either constructive or destructive interference is created with an external light source. This allows each element to switch from reflective to absorbing, from green to black, for example. Each image pixel is composed of tens-to-hundred of bridge elements, which facilitates grey scale and reduces yield requirements. Iridigm's displays are fabricated on glass substrates using standard thin film transistor (TFT) materials and processing techniques, that allow them to construct aluminum bridge elements and a proprietary thin-film stack to control interference. However, because Iridigm's display is based on interference patterns it will be sensitive to viewing angle and will have difficulties achieving paper white quality.
A number of electromechanical shutter bistable display technologies have been pursued and patented for direct view displays, but have not yet succeeded to large scale commercialization due to a variety of issues including fabrication, stiction, limited contrast ratio, poor optical efficiency, high cost and poor pixel uniformity. These displays are bistable in the sense that they are black and white and do not require continuous refresh. However, they do require power to store the image and thus are not bistable in the truest sense of the meaning.
U.S. Pat. No. 3,553,364 to Lee entitled "Electromechanical Light Valve" describes an electromechanical light valve in an array of many such valves for controlling the transmission of light in continuously changing patterns. Each light valve consists of a housing having grounded conducting walls for shielding the interior thereof from external electrostatic forces produced by surrounding valves and a leaf shutter mounted in the housing. The application of a voltage to the leaf shutters causes the shutter to be attracted to the grounded conducting walls. As the voltage differential increases, the angle the shutter deflects increases, which in turn allows less light to pass through the housing.
Lee's design always involves the leaf shutters touching one surface or another, e.g. the conductive center plate or the grounded conductive walls, which can and will cause stiction due to the Van der Waals forces. The optical efficiency of this design is very low due to the low open aperture caused by the opaque conductive sidewalls. The portion of each pixel that is transparent and thus able to transmit light is a small fraction of the pixel. In addition, the cost and complexity of fabricating an array of such housings makes high resolution displays impractical.
U.S. Pat. No. 4,564,836 to Vuilleumier et al. entitled "Miniature Shutter Type Display Device with Multiplexing Capability" describes a display device comprising an insulating carrier and shutters that are capable of rotating under the effect of an electric field. The shutters are grouped in pairs and are controlled by applying a voltage between the shutter and a counter-electrode. After actuating the selected shutter, a holding voltage is then applied between the pair of shutters to hold them in place. Vuilleumier's device involves shutters touching each other or a stop, which can cause stiction problems. This design, like that of Lee, also has low optical efficiency due to the opaque sidewalls of the individual cavities.
U.S. Pat. 5,784,189 to Bozler et al. entitled "Spatial Light Modulator" discloses a spatial light modulator formed of a moveable electrode which is disposed opposite a fixed electrode, and is biased to roll in a preferred direction upon application of an electric field across the electrodes to produce a light valve or light shutter. As shown in FIGS. 24-25, Bozler teaches a quasi "bistable" device, one in which the "hold-on" voltage is less than the voltage required to initially turn the shutter on. Bozler's devices do not exhibit true bistability in the sense that if the field is removed entirely the device does not remain in one of two stable positions. Energy is required to resist the spring force. This consumes power equal to the product of the hold-on voltage and the parasitic resistance.
Bozler's quasi "bistability" is created by forming a step S in the moveable electrode, which produces a hysteresis in the voltage required to unroll the coil. A second way to create a quasi bistable device is to use the Van der Walls forces, which occur when two materials come into contact. By selecting materials and controlling the surface condition a magnitude of adhesion force can be achieved which is low enough to allow roll-up of the shutter at zero applied voltage but large enough to significantly reduce the hold voltage below the roll out voltage, assuming there is no step. An alternate bistable device is illustrated in FIG. 27, in which a deformable membrane switch switches between an up position where it looks white or the color of the conductor and a flat position where it looks black or blue. The deformable switch is biased in the up position by anisotropic stress. Once pulled down by the application of an electric field, the switch will stay down due to the Van der Waals forces as long as the applied electric field exceeds a threshold. In all cases, Bozler's designs cannot achieve true bistability since none of his designs teach a method of actively driving the shutter into both the "ON" state and the "OFF" state.